Heave-damped caisson vessel

ABSTRACT

A floating structure includes an elongate caisson hull and at least one plate set coupled to the hull. The plate set includes multiple heave plates located about an outer edge of the hull so as to form a discontinuous pattern generally symmetric about a vertical axis of the hull.

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates generally to floating structures. Morespecifically, the invention relates to a deep draft caisson vessel forsupporting a deck or other superstructure above a water surface.

[0003]2. Background Art

[0004] The offshore oil industry employs a variety of floatingstructures to perform oil exploration, drilling, and production. Onetype of floating structure is the deep draft caisson vessel, also knownas a spar platform. The performance of the spar platform is governedprimarily by mass distribution on the platform. FIG. 1 illustrates ageneralized spar platform 100 which includes an elongate caisson hull102 supporting a deck 104 above a water surface 106. Mooring lines 108are attached to the hull 102 below the water surface 106 to performstation keeping. Marine risers 110 extend downward from the hull 102 andcarry fluids to and from the platform 100. The hull 102 includes anupper buoyant portion 112, an elongated middle portion 114, and anegatively buoyant lower portion 116. The upper portion 112 provides themajority of the buoyancy required to support the weight (W) of the deck104 above the water surface 106. The lower portion 116 provides a largenegatively buoyant mass (M) to counterbalance the weight (W) of the deck104. The middle portion 114 provides an extended separation (X) betweenthe deck weight (W) and the negative mass (M).

[0005] The combined arrangement of the amount of negative mass (M) andthe distance of separation (X) positions the combined center of gravity(CG) a distance (Y) below the combined center of buoyancy (CB) for theplatform. This arrangement provides desirable stability and pitchcharacteristics. The middle portion 114 increases the platform draft(T). The deep draft moves the keel of the hull 102 below a majority ofwave-induced loading. The in-place draft is typically in excess of 600feet. The deep draft also provides a desirable heave natural period byincreasing the total mass, i.e., the displacement and virtual mass ofthe platform. A typical spar platform design will have a heave naturalperiod in the range of 25-30 seconds. To achieve this the hull of thespar has a large ratio of hull draft (T) to hull diameter (D). A typicalspar hull has a circular cross-section of a diameter between 70 and 150feet.

[0006] Various designs of spar platforms are known in the art. Onedesign is the conventional or classic spar. FIG. 2A illustrates aprior-art classic spar 200 which includes an elongate caisson hull 202and deck 204. The hull 202 has an upper buoyant portion 206 made ofvariable ballast tanks 208 and permanent buoyancy tanks 210. A centerportion 212 of the hull 202 includes an open framed construction whichtraps a large mass of seawater 214. A lower portion 216 of the hull 202contains an amount of fixed ballast 218 which places a center of gravity(CG) of the spar 200 below a combined center of buoyancy (CB) of thespar 200. The fixed ballast 218 generally includes iron ore or otherdense material. The amount of fixed ballast 218 is typically on theorder of 0.5 to 1.0 times the deck weight (W). Mooring lines 220 areattached to the hull through fairleads 222. Helical strakes 224 extendoutward from the hull 202. The helical strakes 224 are made of flatplates which extend in a spiral pattern downward along the hull 202.Apertures 226 (shown in FIG. 2B) in the helical strakes 224 permitpassage of the mooring lines 220 downward along the hull 202 to thefairleads 222. Risers 228 supported by multiple air cans 230 aredisposed within an open well-bay 232 (shown in FIG. 2B) extendingthrough the upper portion 206 of the hull 202. The risers 238 exitthrough guide tubes 234 in the lower portion 216 of the hull 202.

[0007] The deep draft of the classic spar 200 acts to increase the heavenatural period of the deck 204 to a favorable region of 25 to 30seconds. This is achieved primarily from the increase in virtual mass(M_(v)) of the structure due to the large mass of water 214 trappedwithin the center portion 212 of the hull 202. The extended length ofthe hull 202 is therefore required. For heave motions having periods onthe order of heave natural period, the trapped water 214 moves with thehull 202, effectively increasing the apparent total mass of the spar200. For longer period motions, such as 100 seconds or more, the watermay flow in and out of the hull 202. Therefore, the water does notincrease the actual mass of the spar 200 for static and quasi-staticdisplacements. The deep draft also lowers the keel draft (T_(k)),resulting in a reduced wave loading in the vertical direction.

[0008] The resulting spar 200 has very limited wave-induced heavemotions.

[0009] Generally speaking, heave motions are made of two components, asillustrated in FIG. 5A. The first component is a wave period response,referred to as first order motions. This component includes motionsoccurring near the peak wave period of the sea in which the spar islocated, typically 14-16 seconds for the maximum design hurricaneconditions. The magnitude of the first order motions are generallyproportional to the magnitude of wave loading at the keel of the hull(T_(k)). The second component is a long period response, referred to assecond order motions. This component includes motions occurring near tothe natural period of the floating structure in heave. The magnitude ofthe second order motions are generally proportional to the dampingprovided by the hull in the heave direction.

[0010] The classic spar has an extremely deep keel draft (T_(k)),typically set at 650 ft for a wide range of hull diameters,displacement, and deck weights. This depth is generally located below asmuch as 98% or more of the hydrodynamic wave forces for most sea states,as indicated by the magnitude of the wave profile at the keel draft(T_(k)). The result is extremely small first order heave motions. Infact, there may be only negligible first order heave motion for seastates lower than annual storm conditions. The damping of the spar inheave, however, is generally low. This results in comparatively largersecond order motions during storm conditions. The combined heave motionsare still generally quite favorable and are dominated by slow,long-period heave.

[0011] One concern for the in-place performance of a classic spar arisesfrom its continuous, circular cross-sectional shape. A long slender bodyoriented vertically in an ocean current may be susceptible to theformation of vortices along the length of the hull. The vortices willinduce periodic, potentially large magnitude horizontal excursions ofthe floating structure. Referring to FIG. 2A, the helical strakes 224are designed to impede the formation of vortices. The helical strakes224 are generally oriented at an angle approximately 30 degrees fromvertical. The strakes 224 are arranged such that they spiral down thelength of the hull 202. The net affect is to disrupt the horizontal flowof water, imparting some vertical component to the flow. This actiontends to disrupt the formation of vortices. The size of each strake 224may be on the order of 10% of the diameter of the hull 202. Each strake224 may extend 10-15 feet outward from the hull 202. The helical strakes224, therefore, increase the drag of the hull 202 to ocean currents andincrease the requirements of the mooring system. Also the strakes 224interfere with the mooring lines 220, requiring the addition of multipleapertures 226 (shown in FIG. 2B) through the strakes 224. In general,despite the negative effects, strakes 224 are considered as essential tothe classic spar.

[0012] The construction and installation of a classic spar design iseased by its continuous cross-section but complicated by the extendedhull length and anti-vortex strake. The classic spar is generallyconstructed in a horizontal orientation. A support cradle is adapted tosupport the circular sections of the hull on the construction ways. Theheight of the support cradle must be high enough such that anti-vortexstrake clears the ground. The cost of elevated construction, however,may favor the alternative of leaving the strake off on the bottom sideof the hull at the sacrifice of hull symmetry and strake effectiveness.The continuous hull cross-section allows for generally level horizontalfloatation of the hull at a relatively shallow draft. The hull couldtherefore be launched from a construction ways. The circular shape,however, is unstable in roll. Consideration must be taken to augmentroll stability to allow horizontal floatation. In general, however, thehull may not be fabricated within wet towing distance of theinstallation site. Therefore, the hull must be loaded onto a transportvessel.

[0013] The spar hull is generally not considered viable fortransportation with a launch barge, due to the bending moment inducedwhen the hull is launched from the barge once near the installationsite. Instead, a semi-submersible transport ship is employed with thehull floated off near the installation site. The overall length of theclassic spar hull may approach 700 feet, adding the length due to freeboard and deck support legs. This length generally exceeds the capacityof existing semi-submersible transport vessels. As previously discussed,the length of the hull is generally set to achieve the desired heavenatural period and is not subject to being shortened. Therefore, theclassic spar is typically constructed in two pieces. The individualpieces are transported to a location near the installation site. Therethe pieces are floated off for mating either in a dry dock or offshore.The two pieces are mated and welded together. The completed hull is thenwet towed to the installation site where the hull is upended, moored,and the deck installed. The added cost and time due to the hull matingprocedure and dual transportation can be substantial.

[0014] An alternative spar design is a truss spar. FIG. 3 illustrates aprior art truss spar 300 which includes a hull having an upper buoyantportion 302 and a lower portion 304. The lower portion 304 includes anamount of fixed ballast 306. The center portion of the truss spar 300includes an open truss section 308 coupled between the buoyant portions302, 304. Large horizontal plates 310, called heave plates, are locatedat various elevations along the length of the truss 308. These heaveplates 310 act to impede the flow of water along a vertical axis of thetruss spar 300 and permit flow of water perpendicular to the verticalaxis. The heave plates 310 force a large percentage of the water trappedbetween the plates and buoyant portions 302, 304 to move with the sparfor heave motions having periods on the order of the heave naturalperiod. The net effect of the heave plates 310 is to provide virtualmass (M_(v)) in the heave direction. The heave plates 310 also providedynamic damping forces. In contrast to the classic spar design whichemploys trapped water, there is some movement of water around the heaveplates 310, even for shorter period heave motions. The net effect is theaddition of velocity-dependent damping forces.

[0015] One generally desirable characteristic of a truss construction isthat it is not susceptible to vortex formation. The individual trussmembers will form local vortices disrupted at the nodes and combining tonegate the formation of global vortices. As illustrated, the result is areduced requirement for anti-vortex strakes 312 (compare to strakes 224in FIG. 2A which extend along the length of the hull 202 for the classicspar). In general, strakes 312 are only necessary in the upper buoyantportion 302, thereby limiting the horizontal drag increase and reducingthe interference between the mooring lines 314 and strake 312.

[0016] One generally undesirable consequence of replacing the centerportion of the truss spar 300 with a truss is that the keel draft(T_(k)) is elevated above the lower buoyant portion 304 upwards to thebottom of the upper buoyant portion 302. A typical keel elevation may bein the range of 180 to 250 feet of water depth. This reduced keel depthis subject to larger wave loading, as indicated by the magnitude of thewave profile at the elevated keel draft (T_(k)). This characteristicacts to increase the first order heave motions for the truss spar 300(see FIG. 5B). A typical truss spar design might be expected toexperience some noticeable heave motion even in fair weather conditions.This more regular motion may result in an increase in fatigue to thebuoyant portions 302, 304, truss section 308, risers 316, and otherassociated structures. In storm conditions, however, the heave plates310 of the truss section 308 provide very large damping forces. Themagnitude of damping forces induced by the heave plates 310 isproportional to the square of the velocity of the waves. Therefore,under the increased amplitude and velocity induced by storm sea states,the damping forces induced by the heave plates 310 increasesexponentially so as to greatly reduce the second order heave motions.The net result is maximum heave motions generally on the order of thatfor a classic spar design. The motions, however, are dominated byshorter-period first order heave.

[0017] The construction and installation of a truss spar 300 are easedby the ability to reduce hull length but complicated by the horizontalfloatation characteristics of a discontinuous hull. The use of heaveplates 310 in the truss spar 300, to provide heave virtual mass (M_(v))and heave damping forces, results in a hull design wherein the motioncharacteristics are not entirely dependent upon overall hull length. Thelength of the hull may thereby be reduced to fall within the limits ofexisting semi-submersible transport ships. For certain configurations,this aspect might permit the construction of the hull in one piece. Thehull is, however, of two different construction types, which may requireconstruction at separate fabrication yards. Additionally, the horizontalfloatation characteristics of the hull may act to greatly complicate thefloat-off and other installation procedures. The buoyancy of thehorizontally oriented hull is discontinuous along its length. The resultis a deeper horizontal floatation draft and a natural tendency to floatat an angle. A floatation tank 318 attached to the lower portion 304 ofthe hull 302, may be employed to reduce the floatation angle. Thecombination of increased horizontal floatation draft and angle may,nonetheless, make the float-off procedure infeasible. This would againrequire that the two pieces of the hull be mated offshore as with theclassic spar design.

[0018] As illustrated above, prior-art spar platform designs each havetheir relative advantages and disadvantages which might be susceptibleto combination. One prior-art alternative spar platform design combinesa continuous caisson hull with heave plates. As illustrated in FIG. 4A,the ring-plate spar 400 comprises a hull 402 supporting a deck 404. Thehull 402 comprises a buoyant upper portion 406 and a lower portion 408comprising an amount of fixed ballast 410. There is no central portionof the hull. Instead, continuous circular heave plates 412 are locatedat several elevations along the length of the hull 402. The overalldraft of the hull 402 can be reduced due to the increased added mass(M_(v)) (indicated by the dotted lines) and damping provided by theheave plates 412. Unlike with the truss spar (300 in FIG. 3), the keeldraft (T_(k)) remains at the base of the lower portion 408 of the hull402. The keel draft (T_(k)) can therefore remain deeply submerged belowthe majority of wave loading, as indicated by the magnitude of the waveprofile.

[0019] The construction and installation of the ring-plate spar 400,however, place restrictions on the allowable hull length. As mostclearly shown in FIG. 4B, the heave plates 412 extend radially outwardfrom the caisson hull 402, generally to a diameter 50-100% larger thanthe diameter (D) of the hull 402. This arrangement generally makeshorizontal construction and transportation practically infeasible. Thislimitation eliminates many of the advantages sought by the hybriddesign. The hull 402 must instead be fabricated vertically. Economicalfabrication is generally not considered compatible with elevatedconstruction. Vertical construction may also place an absolute limit onallowable overall hull length. The limited hull length reduces the keeldraft (Tk) at the sacrifice of performance. Another consequence of alimited keel draft (Tk) is a non-linear increase in the amount of fixedballast 410 required to provide adequate stability and pitchcharacteristics. To achieve similar characteristics, the amount of fixedballast 410 may have to be increased to multiples of the deck weight.The large increase in fixed ballast increases the required buoyancy,also increasing the hull steel to provide the buoyancy, further increaseweight. For these and other reasons, vertical construction is generallyconsidered undesirable for elongate structures.

[0020] As can be appreciated from the foregoing discussion of prior artstructures, a spar platform design would be highly desirable whichcombined the advantages of a deep draft caisson and heave plates designwithout sacrificing the advantages of horizontal construction andtransportation. It is highly desirable to employ a caisson hull having adeep keel draft. FIG. 5A illustrates the generalized heave motionresponse of a spar platform having low damping but with deep keel draft,such as the prior-art classic spar platform (200 in FIG. 2A). The lowwave-induced hydrodynamic heave forces result in small first order heavemotions, but the low damping results in large second order heave motionsdespite low wave loading. It is also highly desirable to employ heaveplates in combination with a caisson hull. FIG. 5B illustrates thegeneralized heave motion response of a spar platform having high dampingbut with a shallow keel draft, such as the prior-art truss spar platform(300 in FIG. 3). The damping provided by the heave plates results insmall second order heave motions. Heave plates also provide virtual massto increase the heave natural period or allowing a reduction in hulldraft without performance decrease. The reduced keel draft, however,increases the wave loading and results in relatively large first orderheave motions.

[0021]FIG. 5C illustrates the generalized heave motion response of aspar platform having both high damping and a deep keel draft. A sparplatform successfully combining both the reduced wave loading of a deepkeel draft and the increased damping from heave plates results insuperior overall heave motions, having a relatively small first andsecond order motion response. Prior art structures, however, have faceddifficulties combining these two desirable features. In general, theaddition of heave plates has come at the cost of greatly reduced keelsubmergence. In other configurations, combining a deep draft caisson andheave plates added substantial complications to the fabrication,construction, and installation of the floating structure. Further, thesefloating structures also may encounter difficulty and high cost ininstallation due to multiple section construction and offshore hullmating, or may encounter high elevation vertical construction and theresulting draft limitations.

[0022] SUMMARY OF INVENTION

[0023] In one aspect, the invention relates to floating structure whichcomprises an elongate caisson hull and at least one plate set coupled tothe hull. The plate set comprises a plurality of heave plates locatedabout an outer edge of the hull so as to form a discontinuous patterngenerally symmetric about a vertical axis of the hull.

[0024] In another aspect, the invention relates to a floating structurewhich comprises a deep draft caisson hull having a generally prismaticshape and a plurality of heave plates forming a discontinuous ring abouta circumference of the hull.

[0025] In another aspect, the invention relates to a floating structurewhich comprises a buoyant hull having a diameter and a vertical axis andan array of heave plates attached about the diameter of the hull. Thearray of heave plates fit within an imaginary bounding box in ahorizontal plane centered about a vertical axis of the hull. Thebounding box has sides of length no greater than 120% of the diameter ofthe hull.

[0026] In another aspect, the invention relates to a method ofconstruction which comprises constructing a caisson hull in a horizontalorientation upon a support structure which allows a keel of the hull tobe elevated a distance above a ground level and attaching a plurality ofheave plates to the caisson hull at one or more locations along a lengthof the hull.

[0027] Other aspects and advantages of the invention will be apparentfrom the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0028]FIG. 1 illustrates a generalized profile of a prior art sparplatform.

[0029]FIG. 2A is a vertical cross-section of a prior-art classic spar.

[0030]FIG. 2B is a horizontal cross-section of the prior-art classicspar shown in FIG. 2A.

[0031]FIG. 3 is a vertical cross-section of a prior-art truss spar.

[0032]FIG. 4A is a vertical cross-section of a prior-art ring-platespar.

[0033]FIG. 4B is a horizontal cross-section of the prior-art ring-platespar shown in FIG. 4A.

[0034] FIGS. 5A-5C illustrate generalized heave-direction responseamplitude operators for lightly- and highly-damped caisson vessels ofvarious configurations.

[0035]FIG. 6 is a three dimensional outboard profile of a heave-dampedcaisson vessel according to an embodiment of the invention.

[0036]FIG. 7A is an inboard profile of the heave-damped caisson vesselshown in FIG. 6.

[0037]FIG. 7B is a vertical cross-section of the heave-damped caissonvessel shown in FIG. 7A.

[0038]FIG. 8A is a top view of a fabrication site arrangement for theheave-damped caisson vessel shown in FIG. 7A.

[0039]FIG. 8B is a front view of the fabrication site arrangement shownin FIG. 8A.

[0040] FIGS. 9A-C illustrate a buoyancy diagram, side view, and end viewof a floatation arrangement for a heave damped spar platform inaccordance with an embodiment of the invention.

[0041] FIGS. 10A-H illustrate end views of alternative horizontalarrangements and designs of heave plates in accordance with embodimentsof the invention.

[0042] FIGS. 11A-C illustrate outboard profiles and a top view ofalternative vertical arrangement of heave plates in accordance withembodiments of the invention.

DETAILED DESCRIPTION

[0043] Specific embodiments of the invention will now be described withreference to the accompanying drawings.

[0044]FIG. 6 shows a heave-damped caisson vessel 600 with associatedheave damping structures in accordance with an embodiment of theinvention. The heave-damped caisson vessel 600 comprises an elongatecaisson hull 602. In the illustrated embodiment, the hull 602 has anoctagonal cross-section. The hull 602 is of sufficient length to providea deeply submerged keel draft (T_(k)). A desirable keel draft (T_(k)) istypically in excess of 400 feet to place it below the majority ofhydrodynamic wave forces and may be advantageous to exceed 600 feet incertain embodiments. An array of heave plates 604 is attached to thehull 602. The plate array 604 includes multiple plate sets 606comprising four individual heave plates 608. The individual heave plates608 are generally triangular in shape and located in diagonal comers 610of the octagonal cross-section of the hull 602. The four individualheave plates 608 of each plate set 606 are located at a single elevationand form a discontinuous symmetric pattern about a vertical axis of theheave-damped caisson vessel 600.

[0045] Preferably, the uppermost plate set 606 is set at a depth (T_(p))on the order of two times the maximum wave height. For the Gulf ofMexico design hurricane, for example, this depth may be on the order of140 feet. Cross-braced truss structures 612 reinforce the individualheave plates 608 and cross-connect between individual heave plates 608of vertically-spaced plate sets 606. A desirable vertical spacingbetween plate sets 606 will vary between specific embodiments, but maybe established by conventional model testing and computational methodsfamiliar to the art. Anti-vortex strakes 614 are attached to the hull602 at an elevation above the array of heave plates 604. The strakes 614include flat plates oriented approximately 30 degrees from the verticalof the heave-damped caisson vessel 600. The strakes 614 are also placedin the diagonal comers 610 of the hull 602 and form a symmetric patternabout the vertical axis of the hull 602. The location and discontinuouspatterns of the heave plates 608 and strakes 614 about the circumferenceof the hull 602 leave four clear sides 616 to the hull 602. Mooringlines 618 run downward along the clear sides 616 of the hull 602 andextend out through fairleads 620, which are attached to the hull 602below a water surface 622.

[0046]FIG. 7A shows an inboard profile of the heave-damped caissonvessel 600 (previously shown in FIG. 6). The buoyant hull 602 supports adeck 704 above the water surface 622. The hull 602 includes an upperportion 708, a center portion 710, and a lower portion 712. The upperportion 708 includes variable ballast tanks 714 and permanent buoyancytanks 716. The center portion 710 includes an open framed constructiontrapping a large mass of seawater 718. The lower portion 712 includes anamount of negatively buoyant fixed ballast 720. The amount of fixedballast 720 is arranged such that a combined center of gravity (CG) islocated below a combined center of buoyancy (CB) for the caisson vessel600. Mooring lines 722 are attached to the hull 602 through fairleads724. Riser 726 supported by a plurality of air cans 728 are disposedwithin an aperture 730 extending through the upper portion 708 of thehull 602. The risers 726 exit through guide tubes 732 in the lowerportion 712 of the hull 602. Deck legs 734 connect the deck 704 to thehull 602.

[0047] The array of plate sets 604 is attached to the hull 602. Aspreviously described, each plate set 606 includes four individual heaveplates 608. Each heave plate 608 is a flat construction and has agenerally triangular shape. The heave plates 608 are located in fourdiagonal comers (742 in FIG. 7B) of the octagonal hull 602. In thisarrangement the four individual heave plates 608 of a single plate set606 form a symmetric pattern about a vertical axis of the hull 602bounded by an imaginary square shape. The individual plates 608 of aplate set 606 are located at a single elevation. Multiple plates sets606 are disposed at various elevations along the hull 602. The trussstructure 612 reinforces the individual heave plates 608 andinterconnects individual heave plates 608 of vertically-adjacent platesets 606. The array of heave plates 606 forces a percentage of the watertrapped between vertically-adjacent individual heave plates 608 to movewith the caisson vessel 600 for heave motions on the order of the heavenatural period. The net effect of the array of plate sets 606 is toprovide virtual mass (M_(v)) in addition to that provided by the water718 trapped within the center portion 710 of the hull 602. The heaveplates 608 also provide dynamic damping forces.

[0048] The anti-vortex strakes 614 are attached to the diagonal comers(742 in FIG. 7B) of the hull 602, fitting within the aforementionedimaginary square shape. The strakes 614 include flat-plate constructionsoriented approximately 30 degrees from vertical. The strakes 614 disruptthe formation of vortices along the length of the hull 602 by impartinga vertical component to the horizontal water flow. Water flowing aroundone side of the hull 602 is deflected downward. Water flowing around theother side of the hull 602 is deflected upwards. This disruption in flowimpedes vortex formation. The overall requirement for the strakes 614 isgenerally reduced. The comers of the octagonal hull 602 impede vortexformation to some degree. The array of heave plates 606 and trussstructure 612 further act to impede vortex formation. If required,however, the strakes 614 could be placed at multiple elevations alongthe hull 602 to further disrupt vortex formation.

[0049] The keel draft (T_(k)) of the caisson vessel 600 is deeplysubmerged. In addition to displacement, the total mass of the caissonvessel 600 in heave includes both the added mass (M_(v)) contributed bythe trapped water 718 and the added mass (M_(v)) contributed by theheave plates 608. The net effect is to extend the heave natural periodof the caisson vessel 600 for a given keel draft (T_(k)), or in thealternative allow the keel draft (T_(k)) to be reduced while maintainingan equivalent heave natural period. The motion behavior of the caissonvessel 600 provides the advantages of both deep keel draft and thedamping provided by heave plates, as generally represented previously inFIG. 5C. In contrast to prior-art hybrid spar designs, however,embodiments of the invention may be adapted for horizontal fabricationusing conventional shipyard infrastructure.

[0050]FIG. 8A shows the caisson vessel 600 (previously shown in FIGS.6-7B) at a fabrication site. In FIG. 8B, the hull 602 of the caissonvessel 600 is placed upon a construction ways 804. The construction ways804 are fixed to the ground 806. Launch beams 808 elevate the caissonvessel 600 a distance (X) above the construction ways 804. Theconstruction ways 804 are spaced such that they align with load bearingpoints in the hull 602, typically at bulkheads 810. Individual heaveplates 608 are attached to the diagonal comers 742 of the hull 602 toform plate sets 606. The plate sets (606 in FIG. 8A) are placed atvarious elevations along the length (L) of the hull 602 to form an arrayof plate sets (604 in FIG. 8A). Anti-vortex strakes 614 are alsoattached to the hull 602 in the diagonal comers 742. The hull 602 can beconstructed in this horizontal orientation without the attached heaveplates 608 and strakes 614 interfering with the construction ways 804 orground 806. Mooring line fairleads 620 are attached to the four clearsides 824 of the hull 602. On the clear side 824 nearest to the ground806, the fairleads 620 fit within a gap (G) between launch beams 808.After partial or completed construction, the hull 602 may be launched.

[0051]FIG. 9A shows a generalized floatation graph which illustratesbuoyancy and weight distribution for the heave-damped caisson vessel 600(previously shown in FIGS. 6-8B) in a horizontal floatation condition.The elongate caisson structure of the hull 602 provides continuousbuoyancy. Hull weight is unevenly distributed. Referring to FIG. 9B, themajority of the structural weight of the caisson vessel 600 isattributed to the upper hull portion 708. The center hull portion 710and lower hull portion 712 contribute less structural weight. An amountof fixed ballast 720 is placed in the lower hull portion 712 to achievelevel floatation. The octagonal shape of the hull 602 provides astability to roll motions of the hull 602. The continuous waterplaneprovided by the elongate hull 602 results in a relatively shallowhorizontal keel draft (T_(h)). As previously discussed, the use of heaveplates 608 permits reduction of the keel draft (T_(k)) in the installedvertical orientation, which reduces the overall hull length (L) in thehorizontal floatation orientation. The level floatation characteristic,combined with the shallow horizontal keel draft (T_(h)), and the reducedoverall hull length (L) may be employed to permit transportation withinthe draft and length limits of existing semi-submersible transportships. Further, the launch beams (808 in FIG. 9C) may be retained on thehull 602 to provide a load spreading and bearing surface upon the deckof the transport ship.

[0052] Those skilled in the art will appreciate that other embodimentsof the invention can be devised which are within the scope of theinvention. The following is a discussion of some of those variationswhich are possible while still permitting horizontal fabrication of theheave-damped caisson vessel.

[0053]FIG. 10A shows the hull 602 resting on construction ways 804 andlaunch beams 808. The launch beams 808 elevate the hull 602 above theground 806 a distance (X), as previously described in FIG. 8B. Theextent of elevation (X) varies with the specific infrastructure employby a fabrication yard. For example, the elevation (X) may be as low as 3feet or in excess of 10 feet. In general, to retain economicalconstruction, the elevation (X) is generally to approximately 10% ofhull diameter (D). On the lower side 1010 of the hull 602, this places alimit on the placement of structures, such as heave plates 608 andstrakes 614, so as not to interfere with the ground 806 or constructionways 804. Functional aspects of heave plates 608 and strakes 614 designfurther limit placement by strongly favoring symmetry. Symmetry isgeneral preferable about the vertical axis of the hull (+), such thatplacement is symmetric both the X and Y axis. This shall be referred toas single-axis symmetry. It is further preferable that the X and Y axisare symmetric to each other, such that a 90 degree rotation of the X andY axis would not change the configuration. This will be referred to asdual-axis symmetry.

[0054] To achieve dual-axis symmetry, the lower limit on heave plate 608and strake 614 placement forms an imaginary line which is mirrored onfour sides to form an imaginary square 1016. The comers 1018 of theimaginary square 1016 mark the diagonal comers 742 of the hull 602 forheave plate 608 and strake 614 placement. The flat sides 1022 of theimaginary square 1016 mark the clear sides 824 of the hull 602 forplacement of less obtrusive structures such as fairleads, mooring lines,etc. (not shown) dimensions of the imaginary square 1016 are limited byelevation (X), generally restricted to 10% of hull diameter (D).Preferably, the sides 1022 of the square 1016 are no longer than 1.2(D)for reduced elevations (X). The heave plates 608 and strakes 614 aredesigned to fit within the envelope bounded by the imaginary square1016.

[0055] In alternate embodiments, the hull 602 may have cross-sectionsother than octagonal. Where the hull cross-section is not octagonal, abounding box other than a square may result. In such embodiments,bounding box and symmetry may instead be referenced by tangents to thehull. As illustrated in FIG. 10B, a bounding box 1026 can be formed byfinding the tangent 1028 to a side of the hull and adding allowance forelevation (X) generally up to 10% of hull diameter. Specific embodimentsemploying this reference scheme are illustrated in FIGS. 10C-10H.

[0056]FIG. 10C illustrates an octagonal hull cross-section. The heaveplates 608 may be designed flush, as illustrated by the solid lines,where the construction elevation is substantially zero. Where theconstruction elevation is greater than zero the amount of area occupiedby the heave plates 608 can be increased substantially by outwardextension, as illustrated by the dotted lines 1012. The increased platearea will act to increase the virtual mass provided by the heave platesin the installed condition. Both ground clearance and dual-axis symmetryare maintained in this manner.

[0057] Another feature illustrated in FIG. 10C are perforations 1030drilled through the heave plates 608. These perforations 608 reduce tosome degree the effectiveness of the individual heave plate 608 toproduce virtual mass by allowing water to pass through the heave plate.However, movement of water through the perforations 1030 providesadditional damping forces. The number, sizing, and placement of theperforations 1030 may vary. Advantageous perforation design may bedetermined by conventional model testing and computer modelingtechniques known to the art. In general, it may advantageous to placeperforations 1030 in the heave plates 608 closer to the water surface.The majority of damping is generated by wave action rather than hullmotion. Heave plates 608 farther away from the water surface may be leftwithout perforations to retain full virtual mass. This combinationdesign permits versatility in the design of added mass and damping tosuit the conditions at a particular installation location.

[0058]FIG. 10D illustrates a similar heave plate design, order, andarrangement to that of FIG. 10C as applied to a prismatic hull 1032.Both a flush and extended heave plate 608 configurations can be achievedwith a non-octagonal cross-section while maintaining dual-axis symmetryand ground clearance. Similar principles also apply in application to aGreek cross 1034 hull cross-section as illustrated by FIG. 10E.Additional heave plate area is realized in the recesses 1036 of thediagonal corners 1038 of the Greek cross 1034 geometry. Flush orextended heave plate configurations are achieved without loss ofdual-axis symmetry or ground clearance.

[0059] Dual-axis symmetry may be relaxed for certain embodiments.Especially for applications involving highly direction sea states andcurrents, an embodiment may allow limited dual-axis asymmetry.Single-axis symmetry should be maintained, however.

[0060] Lack of single-axis symmetry will couple heave and pitch motions.The total system mass will be moved away from the vertical axis of thehull and damping forces will apply off center. The net result will bethat heave motions induce pitching moments.

[0061]FIG. 10F illustrates the embodiment of FIG. 10C including anadditional 10%D extension of the heave plates 608 along the X-axis. Thisextension acts to greatly increase heave plate area and the resultingvirtual mass. Ground clearance and single axis symmetry are maintainedin both the flush and extended configurations. The loss of dual-axissymmetry may result in limited coupling of heave and yaw under certainconditions. This performance reduction, however, may be outweighed bythe increased virtual mass and damping provided in a designers judgment.

[0062] A hexagonal hull cross-section loses dual-axis symmetry as well.As illustrated in FIG. 10G, a large plate area can be achieved with ahexagonal hull 1040 cross-sectional area. Both the flush and extendedconfiguration maintains clearance and single-axis symmetry. The extendedconfiguration uses the tangential method to determine the bounding box1026 for heave plate 1012 and strake 1014 placement. Alternativesingle-axis symmetry embodiments may be desirable at a designers option.FIG. 10H illustrates a decagonal hull 1042 cross-section havingrelatively small heave plate area. The bounding box 1026 is determinedusing the tangential method. Such a configuration may be desirable wherethe virtual mass requirement is low but the damping requirement is high.As illustrated, the heave plates 1012 include multiple perforations 1030to increase damping forces.

[0063] Flexibility is also achieved in vertical placement of the platessets (606 shown in FIG. 6-7B). As illustrated in FIG. 11A, theindividual heave plates 608 within each plate set 606 may be placed atmore than one elevation. As shown, each heave plate 608 of a plate set606 is placed a distance (dZ) below the previous plate around thecircumference of the hull 602. The result is a spiral staircase-likearrangement of heave plates 608 forming a helical pattern about the hull602. Such an arrangement may further disrupt vortex formation.

[0064] The individual heave plates 608 themselves might also be orientedaway from horizontal alignment. As illustrated in FIG. 11B, theindividual heave plates 608 are aligned at an angle to the horizontalplane such that water flowing around one side of the hull 602 isdeflected upwards and water flowing around the other side of the hull602 is deflection downwards. In this manner, the individual heave plates608 may further act to disrupt vortex formation. Care should be takennot to couple heave and yaw. The overall array pattern should not form afan pattern causing rotation with heave. In both the embodimentsillustrated in FIGS. 11A and 11B, symmetry about the vertical axis ismaintained as illustrated by the top view of FIG. 11C applicable to bothembodiments.

[0065] The invention may provide general advantages. First, theinvention has relatively small first and second order heave motions. Thecaisson vessel has both a deeply submerged keel draft and heave plates.The deep keel draft reduces wave loading, reducing first order heavemotions. The heave plates provide damping forces to reduce second orderheave motions.

[0066] Second, the invention can be constructed and transportedhorizontally. The heave plates and strakes are placed within a boundingbox at the diagonal corners of the hull. These structures thereby do notinterfere with the ground or construction ways during construction orthe ship deck during transportation. The horizontal construction withlimited hull elevation is generally substantially more economical thanvertical construction.

[0067] Third, the invention achieves a large amount of virtual mass. Theextended hull traps a large amount of water within the hull. The virtualmass provided by the trapped water combines with virtual mass providedby the heave plates. The result is that the vessel can be designed withan elongated heave natural period for a given hull draft.

[0068] Fourth, the invention allows for transportation within the draftand length limits of existing semi-submersible transport ships. Theincreased virtual mass provided by the heave plates allows a reductionin hull length. The continuous caisson hull provides shallow draft,level horizontal floatation. These characteristics combine to fallwithin the allowable limits imposed by existing transport ship designs.

[0069] Fifth, the invention eliminates interferences between the heaveplates, strakes, and mooring lines. The invention provides multiplesides clear of strakes and heave plates. Mooring lines and fairleads mayplaces along the clear sides to avoid interference and reduce designcomplications.

[0070] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A floating structure comprising: an elongatecaisson hull; at least one plate set coupled to the hull, the plate setcomprising a plurality of heave plates located about an outer edge ofthe hull so as to form a discontinuous pattern generally symmetric abouta vertical axis of the hull.
 2. The floating structure of claim 1wherein the individual heave plates are contained within an imaginarybounding box formed by a square centered on a vertical axis of the hullhaving a side length no greater than 120% of a diameter of the hull. 3.The floating structure of claim 1 wherein a cross-section of the hull isoctagonal and the individual heave plates are located at one or moreelevations in four diagonal corners of the hull.
 4. The floatingstructure of claim 1 wherein a cross-section of the hull is Greek crossand the heave plates are located within recesses of the cross.
 5. Thefloating structure of claim 1 wherein the heave plates are located atmore than one elevation.
 6. The floating structure of claim 1 furthercomprising a plurality of support structures reinforcing the heaveplates, the support structures being located at multiple locations alongthe length of the hull and forming a pattern which disrupts the flow ofwater in a horizontal direction so as to impede formation of globalvortices.
 7. The floating structure of claim 1 wherein one or more ofthe individual heave plates further comprise one or more perforationsadapted to increase damping forces provided by the heave plates.
 8. Afloating structure comprising: a deep draft caisson hull having agenerally prismatic shape; a plurality of heave plates forming adiscontinuous ring about a circumference of the hull.
 9. The floatingstructure of claim 8 wherein the discontinuous ring of heave plates apattern generally symmetric about a vertical axis of the floatingstructure.
 10. The floating structure of claim 8 wherein a cross-sectionof the hull is octagonal and the heave plates are located at one or moreelevations in four diagonal corners of the hull.
 11. The floatingstructure of claim 8 wherein a cross-section of the hull is a Greekcross and the heave plates are located within recesses of the cross. 12.The floating structure of claim 8 wherein the heave plates are locatedat more than one elevation.
 13. The floating structure of claim 8wherein the hull has a positively buoyant upper portion and a negativelybuoyant lower portion, such that a combined center of gravity for thefloating structure is located below a combined center of buoyancy forthe floating structure.
 14. The floating structure of claim 8 furthercomprising a plurality of support structures reinforcing the individualheave plates, the support structures are located at located at multiplelocations along the length of the hull and form a pattern adapted todisrupt the flow of water in a horizontal direction so as to impedeformation of vortices.
 15. The floating structure of claim 8 wherein oneor more of the individual heave plates further comprise one or moreperforations adapted to increase damping forces provided by the heaveplates.
 16. A floating structure comprising: a vertical columnar hullhaving a diameter and an outer shell having at least four sides, one ormore heave plates attached to the hull wherein the heave plates arecontained within an imaginary box formed by lines parallel to tangentsto the four sides, wherein each parallel line is located within adistance of 10% of the hull diameter from the tangent.
 17. The floatingstructure of claim 16 wherein a cross-section of the hull is octagonaland the heave plates are located at one or more elevations in fourdiagonal comers of the hull.
 18. The floating structure of claim 16wherein the heave plates form an array comprising one or more platesets, the plates sets comprising a pattern of individual heave platessymmetric about a vertical axis of the floating structure.
 19. Thefloating structure of claim 16 wherein the individual plates of a plateset are located at more than one elevation.
 20. The floating structureof claim 16 wherein the hull has a positively buoyant upper portion anda negatively buoyant lower portion, such that a combined center ofgravity for the floating structure is located below a combined center ofbuoyancy for the floating structure.
 21. The floating structure of claim16 wherein the array of heave plates further comprises a plurality ofsupport structures reinforcing the individual heave plates, the supportstructures adapted to impede formation of vortices induced by movementof water about the hull.
 22. The floating structure of claim 16 whereinone or more of the individual heave plates further comprise one or moreperforations adapted to increase damping forces provided by the heaveplates.
 23. A floating structure comprising: a buoyant hull having adiameter and a vertical axis, and an array of heave plates attachedabout the diameter of the hull, wherein the array of heave plates fitwithin an imaginary bounding box in the horizontal plane that iscentered about a vertical axis of the hull, the bounding box havingsides of length no greater than 120% of the diameter of the hull. 24.The floating structure of claim 23 wherein the bounding box is a square.25. The floating structure of claim 23 wherein the hull has an octagonalcross-section and the heave plates are located at one or more elevationsin diagonal corners of the hull.
 26. The floating structure of claim 23wherein the array of heave plates is composed of one or more plate sets,the plates sets comprising a pattern of individual heave platessymmetric about a vertical axis of the floating structure.
 27. Thefloating structure of claim 23 wherein the individual plates of a plateset are located at more than one elevation
 28. The floating structure ofclaim 23 wherein the hull has a positively buoyant upper portion and anegatively buoyant lower portion, such that a combined center of gravityfor the floating structure is located below a combined center ofbuoyancy for the floating structure.
 29. The floating structure of claim23 wherein the array of heave plates further comprises a plurality ofsupport structures reinforcing the individual heave plates, the supportstructures adapted to impede formation of vortices induced by horizontalmovement of water about the hull.
 30. The floating structure of claim 23wherein one or more of the individual heave plates further comprise oneor more perforations adapted to increase damping forces provided by theheave plates.
 31. A method of construction comprising: constructing acaisson hull in a horizontal orientation upon a support structure whichallows a keel of the hull to be elevated a distance above a groundlevel; and attaching a plurality of heave plates to the caisson hull atone or more locations along a length of the hull.
 32. The method ofconstruction of claim 31 wherein the caisson hull has an octagonalcrosssection and the heave plates are polygonal in shape and adapted toattach in a symmetric pattern in diagonal comers of the octagon withoutinterference with the support structure or ground.
 33. The method ofconstruction of claim 31 wherein the heave plates form an array, thearray comprising one or more sets comprising a plurality of individualheave plates, wherein the individual heave plates form a generallysymmetric pattern about a central axis of the hull.
 34. The method ofconstruction of claim 31 wherein the caisson hull and heave plates areadapted to provide substantially horizontal floatation of the hullsufficient to enable the hull to be launched as a single piece.
 35. Themethod of construction of claim 31 wherein the horizontal floatationoccurs at a draft of less than thirty feet.
 36. The method ofconstruction of claim 31 wherein the keel elevation is less than tenfeet.
 37. A method of increasing the heave natural period of a floatingstructure comprising: attaching a plurality of heave plates to a buoyanthull having a draft in excess of 300 ft, wherein: the heave plates areattached at one or more elevations and arranged so as to form adiscontinuous pattern generally symmetric about a vertical axis of thehull, and the heave plates are located vertically along a length of thehull so as to increase the added mass of the hull in the heavedirection.
 38. The method of claim 31 wherein more than one heave plateis placed in a general vertical line relative to one another, whereinthe heave plates are spaced a distance apart vertically so as toincrease the virtual mass of the floating structure in the heavedirection.
 39. The method of claim 31 wherein the heave plates comprisegenerally flat plates having a substantially horizontal orientation soas to impeded the flow of water in a vertical direction along the hullbut so as to permit the flow of water in the horizontal direction aboutthe hull.
 40. The method of claim 31 wherein a cross-section of the hullis octagonal, and the heave plates comprise triangular geometry and arelocated at one or more elevations in four diagonal corners of the hull.